1. Field of the Invention
This invention provides an improved process for thermally converting volatile fatty acid (VFA) salts (e.g. calcium acetate, propionate, butyrate) to ketones.
2. Review of Related Art
There are two useful methods for producing ketones from fatty acids (Conant and Blatt, "The Chemistry of Organic Compounds," Macmillan Co., New York, 1947). In one method, ketones may be prepared from fatty acids directly by passing them over a catalyst (e.g. MnO or ThO.sub.2) at 300.degree. C., according to Equation (1): EQU 2RCOOH(g).fwdarw.R.sub.2 CO(g)+H.sub.2 O(g)+CO.sub.2 (g) (1)
According to this reaction, pure acetic acid yields only acetone, but a mixture of acids will yield mixed ketones. For example, if acetic acid and propionic acid were fed to such a reactor, the products will be acetone, methyl ethyl ketone, and diethyl ketone.
In an alternative method, ketones are produced from VFA salts, and no catalyst is necessary. Calcium or sodium salts of VFAs decompose at temperatures of 300-400.degree. C., according to exemplary Equation (2): EQU [RCOO].sub.2 Ca(s).fwdarw.R.sub.2 CO(g)+CaCO.sub.3 (s) (2)
This reaction has a "fairly high" yield, as long as the ketone decomposition temperature is not exceeded (Hurd, "The Pyrolysis of Carbon Compounds, "The Chemical Catalog Co., New York, 1929).
In 1834, Peligot heated calcium acetate and obtained acetone (see Mellan, "Ketones," Chemical Publishing Co., New York, 1968). In the latter half of the 19th century, the process was applied industrially to produce acetone, using calcium acetate obtained from the distillation of wood.
Ardagh and his coworkers produced some of the best experimental data available for the decomposition of calcium acetate (Ardagh et al., Industrial and Engineering Chemistry, 16:1133-1139, 1924). They found that the contemporary literature cited a variety of temperatures for decomposition, from 290 to 500.degree. C.; Ardagh, et al., concluded that a temperature of "between 430 and 490.degree. C. is satisfactory," although they found that the reaction actually commences as low as 160.degree. C. They also calculated the yield of acetone from calcium acetate to be 99.5% of the theoretical yield after a 7-hour reaction at 430.degree. C.; after only one hour at this temperature, the yield was 96%. The reaction was somewhat unpredictable, with yields varying by as much as 15% in seemingly duplicate runs. Two primary factors apparently contribute to low yields: presence of oxygen in the reaction vessel and removing the acetone from the hot vessel too slowly.
A French corporation developed an "improved process" for producing 4-heptanone (dipropyl ketone) by pyrolyzing calcium butyrate (Brit. Pat. 216120, Societe Lefranc et Cie, 1925). Their process enabled a recovery of ketone "almost equal to the theoretical yield" given by Equation (2) above. The butyrate salt melts at 360.degree. C., and becomes a "pasty, porous and spongy mass" that conducts heat poorly, and may trap ketone vapors within its pores. To prevent this, the investigators mixed the salt with an inert substance, such as clay or sand, in even proportions, and used a high-speed agitator in the reaction vessel. They found that the reaction began at 300-350.degree. C., and that at 390-400.degree. C., the rate of decomposition was adequate for significant ketone condensation. Whereas a pure salt decomposes to form a single ketone species, a mixture of different fatty acid salts yields a mixture of ketones (Lefranc, Actualites Scientifique et Industrielles, 936:3-27, 1943).
During World War II, a French company constructed a pilot-scale operation using 640 kg/h of acid salts (Depasse, Bull. Assoc. Chim. Sucr. Distill. Fr., 62:317-339, 1945). They found that the reaction must take place in an anhydrous environment, because even the water dislodged from hydrated salts will dissolve some of the lighter ketones, creating a difficult and expensive separation problem. Thorough drying is therefore necessary before commencing with pyrolysis. Another phenomenon is that the mixed-ketone vapor is a complex azeotrope at temperatures below the condensation point of the most volatile ketone (Lefranc, 1943; Depasse, 1945).
Schultz and Sichels (J. Chem. Ed., 38:300-301, 1961) pointed out substantial inconsistency in the body of knowledge on this process. Although many contemporary organic chemistry texts mentioned pyrolytic decomposition of fatty acid salts, indicating that the reaction gives high yields, the text cited relevant literature that gave little specific information about yields and composition of the product.
Because heat transfer from the reactor wall to adjacent solids is very slow, externally firing the reactor to provide reaction heat through the reactor wall is unsuitable for a large industrial process. In addition to requiring a very large reactor, uneven heating and long reaction times result in low yields due to production of side products by degradation of the ketones. Although it is possible to pass hot nitrogen gas through a bed of VFA salt in order to heat the salt for thermally converting a volatile fatty acid salt to a ketone, this approach requires very large heat exchangers to preheat the gas. Similarly, large heat exchangers are required to cool the gas and condense the ketone product. Thus, existing processes for pyrolytic conversion of organic acid salts to ketones are unsatisfactory, and there remains a need for improved processes.